1. Field of the Invention
This invention relates to optical information processors, and more particularly to an optical information processor which employs a semiconductor laser device as a light source.
2. Description of the Prior Art
In recent years, optical information processors which employ semiconductor laser devices instead of gas lasers as the light sources thereof have been vigorously developed. An optical disk is an example of such technique. The optial disk is such that, using the semiconductor laser device, information signals recorded on a disk are played back or information is recorded onto the disk at a high density. In order to record the information signals onto the disk or play them back therefrom with the semiconductor laser, a light beam emergent from the semiconductor laser device must be formed into a light spot of approximately 1 .mu.m in diameter on the disk by the use of a coupling lens and an objective which constitute an optical system. In general, the semiconductor laser device has a light emitting region which is not square but rectangular, and hence, the beam divergence parallel to the junction of the laser device is unequal to that perpendicular thereto. To the end of forming an isotropic or circular spot on the disk by means of such a semiconductor laser device, it is necessary to make the numerical aperture of the coupling lens small and to use only the beam in the vicinity of the optic axis of the optical system so as to render uniform the intensity distribution of light emergent from the coupling lens.
With this measure, however, only part of the light beam emitted from the semiconductor laser device is projected onto the disk, resulting in the disadvantage that the efficiency of light utilization of the apparatus is poor. That is, the ratio of the intensity of the focused light on the disk to that of the light emitted from the diode laser is small. Especially in case of the recording, a metallic thin film in the disk must be molten to form holes, and a light intensity several times higher than in case of the playback is therefore required. In addition, the semiconductor laser device has its lifetime shortened when it produces a light quantity exceeding a certain fixed value. In the optical information processor employing the semiconductor laser device, accordingly, it is necessary by all means from the viewpoints of lifetime and reliability that the efficiency of utilization of the light of the laser device is enhanced to restrain the optical output of the laser device to the utmost.
The disadvantages above described will be explained in detail in conjunction with a prior-art apparatus. As stated before, since the semiconductor laser device generally has the rectangular light emitting region, the beam divergence is anisotropic. The angle of divergence of the semiconductor laser beam differs depending upon the structure of the semiconductor laser device. As illustrated in FIG. 1, let .theta..parallel. and .theta..perp. denote the respective angles at e.sup.-2 parallel to the junction of the laser device and perpendicular thereto in the intensity distribution of the laser beam in the far-field pattern. Then, in the CSP (channeled-substrate-planar) type semiconductor laser, EQU .theta..parallel.=8.degree., .theta..perp.=24.degree. and .theta..perp./.theta..parallel.=3 (1)
In the BH (buried-heterostructure) type semiconductor laser, EQU .theta..parallel.=16.degree., .theta..perp.=32.degree. and .theta..perp./.theta..parallel.=2 (2)
In the BH type laser, the ratio .theta..perp./.theta..parallel. of the beam divergence angles is 2, while in the CSP type laser, it is 3. The axis of abscissas in FIG. 1 represents the angle of divergence, and the axis of ordinates the intensity of light. FIG. 2 shows an example of a prior-art optical information processor for forming the isotropic or circular spot of the diameter of approximately 1 .mu.m on the disk in case where the cross-section of the light beam of the semiconductor laser device is anisotropic or elliptic.
Referring to FIG. 2, a beam having an elliptical beam divergence as has emerged from one facet of a semiconductor laser device 1 is shaped into a light spot 5 on a disk 4 by a coupling lens 2 and an objective 3. A light detector 6 is means for detecting the optical output of the semiconductor laser device 1. Shown at A is an optical axis. In FIG. 2, the numerical aperture NA of the coupling lens 2 has the following relation where 74 denotes the half solid angle defined between the semiconductor laser 1 and the lens 2: EQU N A=sin .theta. (3)
As regards the beam divergence of the semiconductor laser device 1, when the magnitudes at e.sup.-2 parallel to the junction and perpendicular thereto are denoted by .theta..parallel. and .theta..perp. respectively as described above, the numerical aperture NA Of the coupling lens 2 must be selected as follows in order to form the circular spot 5 on the disk 4 by the use of such semiconductor laser device: EQU .theta..ltorsim..theta..parallel.&lt;.theta..perp. (4)
That is, it is necessary that the numerical aperture of the coupling lens 2 is made small to intercept light rays outside the axis and to use the beam only in the vicinity of the optical axis A (.theta.=0) so as to make uniform the intensity distribution of light emergent from the coupling lens 2. According to the beam divergence angles shown in FIG. 1 and Expressions (1), (3) and (4), the following is assumed in the CSP type laser: ##EQU1## Then, the beam having passed through the coupling lens 2 becomes substantially circular, so that the circular spot 5 is formed on the disk 4.
However, when the light rays outside the optical axis are shut off in this manner, only part of the light beam emitted from the semiconductor laser device is projected onto the disk, which results in the disadvantage that the efficiency of utilization of the light of the laser device is inferior.
Usually, the disk rotates while moving up and down to the extent of 1 mm. In order to prevent the diameter of the spot from changing in spite of the vertical motions of the rotating disk, auto-focusing needs to be performed by optically detecting a deviation signal of the focused spot from the disk surface.
In the construction shown in FIG. 2, when the light reflected from the disk 4 is fed back to the semiconductor laser device 1, the output of the semiconductor laser 1 varies in accordance with the amount of the reflected light from the disk surface, so that the information of the disk 4 can be reproduced with the output of the light detector 6. This technique is disclosed in U.S. Pat. No. 3,941,945.
On the other hand, Japanese Unexamined Patent Application Publication No. 53-17706 has proposed a technique wherein to the end of detecting the deviation of a light beam on a disk, a light source or a lens is wobbled in the direction of the optical axis thereof, and the laser output is synchronously detected. This technique, however, has the disadvantage that the detectable range of deviations for the auto-focusing control is narrow. FIG. 3 shows the output variation of the semiconductor laser device 1 at the time when, in the construction shown in FIG. 2, the numerical aperture NA of the coupling lens 2 was 0.1 and the disk 4 was minutely moved along the optical axis. As understood from FIG. 3, when the numerical aperture NA of the coupling lens 2 is as very small as 0.1, the detectable range of deviations for the auto-focusing control extends only 10 .mu.m. This drawback is attributed to the fact that the focus position of the reflected and fed-back light beam near the facet of the laser device changes greatly due to the movements of the disk. That is, the defocusing of the fed-back light spot on the facet of the laser device as is ascribable to the deviation of the focused spot is conspicuous, with the result that the detectable range of deviations for the auto-focusing becomes as small as 10 .mu.m. In this manner, the optical information processor in which the light reflected from the disk is fed back to the semiconductor laser device has the disadvantage of the small detectable range of deviations. This has led to the problem that the auto-focusing control is difficult and that information cannot be reproduced from a disk of great vertical movements.
On the other hand, the use of a cylindrical lens has been considered in order to cause a spot focused on a disk to approximate a circle. The cylindrical lens, however, is disadvantageous in that a high machining precision is difficult to be attained, resulting in a high cost, and that the arrangement of an optical system becomes complicated. It is also difficult to converge a light spot into a circular one of 1 .mu.m, because the astigmatism is greatly influential on account of the use of the cylindrical lens.